Sintered bearing and manufacturing method for same

ABSTRACT

Provided is a sintered bearing ( 1 ), including 3 to 12% by mass of aluminum, 0.05 to 0.5% by mass of phosphorus, and the balance including copper as a main component, and inevitable impurities, the sintered bearing ( 1 ) having a structure in which an aluminum-copper alloy is sintered with a sintering aid added to raw material powder, a pore (db, do) in a surface layer portion of the sintered bearing ( 1 ) being formed smaller than an internal pore (di).

TECHNICAL FIELD

The present invention relates to a sintered bearing that is excellent incorrosion resistance and abrasion resistance and has high strength, andto a manufacturing method for the sintered bearing.

BACKGROUND ART

Hitherto, for example, in an engine using gasoline or light oil as fuel,a motor-type fuel pump has been used. In recent years, engines includinga motor-type fuel pump using fuel such as gasoline or light oil havebeen widely used in various parts of the world. Gasoline, light oil, andthe like of a variety of quality have been used in regions of the world,and gasoline of inferior quality has been used in many regions. As onekind of gasoline of inferior quality, gasoline containing an organicacid is known. In the case of using a copper-based sintered bearing in amotor-type fuel pump, the copper-based sintered bearing is corroded withthe organic acid contained in such gasoline of inferior quality. Thiscorrosion occurs on the periphery of an opening of a pore that is openedon a bearing surface and an inner surface of the pore, and furtheroccurs on an inner surface of a pore that is present inside the bearingand communicates with the inside from the surface, for example. Thisdegrades the strength of the bearing, with the result that the life ofthe copper-based sintered bearing becomes short.

Further, in recent years, engines for automobiles and the like are beingminiaturized and reduced in weight actively. Along with this, there isalso a demand for miniaturization and reduction in weight of fuel pumps,and sintered bearings to be incorporated in the fuel pumps are alsorequired to be compact. For example, in order to miniaturize amotor-type fuel pump while ensuring discharge performance thereof, it isnecessary to increase the number of rotations. Along with this, fuelsuch as gasoline introduced into a fuel pump passes through a flow pathof a narrow gap at high speed. Under such a condition, a sinteredbearing is required to have further high strength and abrasionresistance as well as compactness. Therefore, although a related-artcopper-based sintered bearing has high strength, the abrasion resistancethereof is not sufficient.

As a sintered bearing to be used for such a purpose, for example, PatentDocument 1 discloses a Cu—Ni—Sn—C—P-based sintered bearing.

On the other hand, as a sintered bearing excellent in mechanicalcharacteristics and corrosion resistance, an aluminum bronze-basedsintered bearing is known. This sintered bearing has a problem in thataluminum oxide covering the surface of aluminum-containing copper alloypowder to be generated during sintering inhibits sintering, and hence asintered compact having sufficient strength cannot be obtained easily.Patent Document 2 discloses a technology regarding mixed powder forsintered aluminum-containing copper alloy and a manufacturing method forthe mixed powder in order to solve the above-mentioned problem.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP 4521871 B2-   Patent Document 2: JP 2009-7650 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the Cu—Ni—Sn—C—P-based sintered bearing disclosed in Patent Document1, although strength and abrasion resistance are enhanced, corrosionresistance cannot be considered to be sufficient. Further, the sinteredbearing contains Ni, which is a rare metal, and hence involves a problemin terms of cost as well.

The aluminum-containing copper alloy powder disclosed in Patent Document2 is excellent in forming ability and sintering property. However, afurther study is necessary in order to obtain a product suitable formass production satisfying stable mechanical characteristics,compactness, and reduction in cost as an aluminum bronze-based sinteredbearing using the aluminum-containing copper alloy powder.

In view of the problems of the related art, it is an object of thepresent invention to provide an aluminum bronze-based sintered bearingin which corrosion resistance, and mechanical characteristics such asstrength and abrasion resistance are enhanced, and compactness andreduction in cost are realized and to provide a manufacturing method forthe aluminum bronze-based sintered bearing, which has good productivityand low cost, and thus is suitable for mass production.

Solutions to the Problems

The inventors of the present invention variously studied so as toachieve the above-mentioned object, and as a result, reached a novelconcept of effectively using the expansion by sintering in order toenhance a bearing function and realize compactness and reduction in costin an aluminum bronze-based sintered bearing, and a novel concept ofeffectively using the expansion by sintering in order to realize amanufacturing method that has good productivity and low cost, and thusis suitable for mass production, as a manufacturing method for thealuminum bronze-based sintered bearing.

A sintered bearing according to one embodiment of the present inventionas a technical solution for achieving the above-mentioned object is asintered bearing, comprising 3 to 12% by mass of aluminum, 0.05 to 0.5%by mass of phosphorus, and the balance comprising copper as a maincomponent, and inevitable impurities, the sintered bearing having astructure in which an aluminum-copper alloy is sintered with a sinteringaid added to raw material powder, a pore in a surface layer portion ofthe sintered bearing being formed smaller than an internal pore. Thus,corrosion resistance, mechanical characteristics such as strength andabrasion resistance, oil film formation property, and oil holdingproperty can be enhanced, and compactness and reduction in cost can berealized.

Further, a manufacturing method for a sintered bearing according to oneembodiment of the present invention is a manufacturing method for asintered bearing comprising 3 to 12% by mass of aluminum, 0.05 to 0.5%by mass of phosphorus, and the balance comprising copper as a maincomponent, and inevitable impurities, the manufacturing methodcomprising at least: a green compact forming step of forming a greencompact in which a sintering aid is added to raw material powder; asintering step of obtaining, from the green compact, a sintered compacthaving a structure in which an aluminum-copper alloy is sintered; and asizing step of subjecting the sintered compact to dimension shaping.Thus, it is possible to achieve a manufacturing method for the aluminumbronze-based sintered bearing, which has good productivity and low cost,and thus is suitable for mass production. In the sintered bearingmanufactured by the manufacturing method, corrosion resistance,mechanical characteristics such as strength and abrasion resistance, oilfilm formation property, and oil holding property can be enhanced, andcompactness can be realized.

It is preferred that the content of aluminum be from 3 to 12% by mass.It is not preferred that the content of aluminum be less than 3% bymass, because the effects of corrosion resistance and abrasionresistance as the aluminum bronze-based sintered bearing are notobtained, and the expansion of the sintered compact is small. On theother hand, it is not preferred that the content of aluminum be morethan 12% by mass, because sintering does not occur easily and thesintered compact expands too much.

It is preferred that the blending amount of phosphorus be from 0.05 to0.5% by mass. It is not preferred that the blending amount of phosphorusbe less than 0.05% by mass, because the sintering accelerating effectbetween a solid phase and a liquid phase is unsatisfactory. On the otherhand, it is not preferred that the blending amount of phosphorus be morethan 0.5% by mass, because sintering proceeds excessively, and aluminumis segregated to increase the precipitation of a γ-phase, with theresult that a sintered compact becomes brittle. In an alloy phase ofcopper and aluminum, a β-phase is changed to a γ-phase at eutectoidtemperature (565° C.).

Sintering can be accelerated to enhance strength by adding 1 to 4% bymass of silicon and 0.5 to 2% by mass of tin as the sintering aid to theraw material powder.

It is preferred that the blending amount of silicon be from 1 to 4% bymass. When the blending amount of silicon is less than 1% by mass, theamount of a liquid phase to be generated is small, and the liquid phasesintering acceleration effect at low temperature becomes insufficient,with the result that a sintered compact that is dense and hasappropriate hardness cannot be obtained. On the other hand, it is notpreferred that the blending amount of silicon be more than 4% by mass,because a sintered compact to be obtained is hard and brittle.

It is preferred that the blending amount of tin be from 0.5 to 2% bymass. It is not preferred that the blending amount of tin be less than0.5% by mass, because the effect of raising a green compact density bythe addition of tin powder cannot be obtained. On the other hand, it isnot preferred that the blending amount of tin be more than 2% by mass,because a high concentration of tin is precipitated in a grain boundary,which degrades the quality of outer appearance of a sintered compact.

Further, when a total of 0.05 to 0.2% by mass of aluminum fluoride andcalcium fluoride is added as the sintering aid to a total of 100% bymass of raw material powder comprising aluminum, phosphorus, and thebalance comprising copper as a main component, and inevitableimpurities, the sintering aid can react with aluminum oxide to begenerated during an increase in temperature of sintering to break analuminum oxide film, thereby accelerating sintering. It is preferredthat the total blending amount of aluminum fluoride and calcium fluoridebe from 0.05 to 0.2% by mass. When the total blending amount of aluminumfluoride and calcium fluoride is less than 0.05% by mass, the effect asthe sintering aid is insufficient, and a sintered compact that is denseand has appropriate hardness cannot be obtained. On the other hand, whenthe total blending amount of aluminum fluoride and calcium fluoride ismore than 0.2% by mass, the effect as the sintering aid is not enhancedeven when the blending amount is increased any more, and hence it ispreferred that the total blending amount of aluminum fluoride andcalcium fluoride be limited to 0.2% by mass or less from the viewpointof cost.

When the aluminum fluoride and calcium fluoride are used as thesintering aid, it is preferred that zinc be added in an amount of from 2to 4% by mass. Zinc has a low melting point, and accelerates thesintering of copper and aluminum and further the diffusion of aluminum.Zinc is further excellent in corrosion resistance. When the blendingamount of zinc is less than 2% by mass, the accelerating effects on thesintering of copper and aluminum and the diffusion of aluminum cannot beobtained. On the other hand, it is not preferred that the blendingamount of zinc be more than 4% by mass, because zinc evaporates tocontaminate a sintering furnace during sintering, and aluminum issegregated to be prevented from being diffused.

Further, it is preferred that silicon be added in an amount of from 0.5to 3% by mass. Silicon generates a copper silicon-based liquid phasewith respect to a phase of inhibiting the progress of sintering formedduring a sintering step, thereby accelerating sintering. Siliconaccelerates the diffusion of aluminum during the sintering step andhence reduces the amount of aluminum to decrease a γ-phase. It ispreferred that the blending amount of silicon be from 0.5 to 3% by mass.When the blending amount of silicon is less than 0.5% by mass, theaccelerating effect on the diffusion of aluminum during the sinteringstep is insufficient, and along with this, the decreasing effect on aγ-phase becomes insufficient. On the other hand, when the blendingamount of silicon is more than 3% by mass, silicon effects a reactionduring an increase in temperature of sintering to oxidize aluminum inblack, which causes inconvenience of discoloration.

It is preferred that the raw material powder of copper mainly compriseelectrolytic copper powder. The electrolytic copper powder has adendritic shape. Therefore, the electrolytic copper powder can diffusealuminum sufficiently into copper, and is excellent in forming ability,sintering property, and sliding characteristic.

1 to 5% by mass of graphite may be added with respect to a total of 100%by mass of the raw material powder and the inevitable impurities. Thus,graphite is present as free graphite in dispersed and distributed poresand imparts excellent lubricity to a sintered bearing. As a result,abrasion resistance can be further enhanced. It is preferred that theblending amount of graphite be from 1 to 5% by mass. When the blendingamount of graphite is less than 1% by mass, the enhancing effects onlubricity and abrasion resistance through the addition of graphitecannot be obtained. On the other hand, it is not preferred that theblending amount of graphite be more than 5% by mass, because strength isdecreased.

It is preferred that the sintered bearing have a compressed layer in asurface layer, the compressed layer have a density ratio α1 higher thana density ratio α2 of the inside, the density ratio α1 be 80%≦α1≦95%,and a ratio T/D1 of an average value T of depths of the compressed layerto an inner diameter dimension D1 of the bearing surface be 1/100≦T/D1≦1/15. Herein, the density ratio α is represented by the followingexpression:

α(%)=(ρ1/ρ0)×100

where ρ1 represents a density of a porous body, and ρ0 represents adensity in the case where it is assumed that the porous body has nomicropores.

According to the above-mentioned configuration, mechanicalcharacteristics such as strength and abrasion resistance can beenhanced, and corrosion resistance, oil film formation property, and oilholding property can be enhanced in a green compact design.

It is preferred that, in an outer surface of the sintered bearing, apore on a bearing surface be formed larger than a pore on the remainingouter surface. Thus, corrosion resistance and oil film formationproperty can be enhanced on the bearing surface on a radially innersurface side, and on the other hand, corrosion resistance and oilholding property can be enhanced on a radially outer surface and an endsurface close to a closed state.

A satisfactory lubrication state can be obtained from the beginning ofan operation by using an oil-impregnated bearing as the sinteredbearing. As a lubricant, mineral oil, poly-α-olefin (PAO), an ester,liquid grease, or the like can be used.

As a sintered bearing for a fuel pump, it is preferred that the contentof aluminum be from 8 to 9% by mass. This suppresses sulfidationcorrosion and organic acid corrosion caused by gasoline of inferiorquality, and the sintered bearing thus obtained is excellent inperformance such as initial conformability and durability.

It is preferred that, in the manufacturing method for a sinteredbearing, a forming in the sizing step comprises a die, a pair ofpunches, and a core, and the sizing step comprises compressing thesintered compact from both sides in an axial direction and a radiallyouter side with the punches and the die to shape a radially inner sideof the sintered compact with the core. Thus, the sintered bearing can besubjected to dimension shaping and desired pores can be formed througheffective use of the expansion of an aluminum copper-based sinteredbearing by sintering.

It is preferred that the size of each pore on the surface of thesintered compact can be set by adjusting a dimension difference betweenan inner diameter dimension of the die and an outer diameter dimensionof the sintered compact and a dimension difference between an outerdiameter dimension of the core and an inner diameter dimension of thesintered compact. Thus, the size of each pore on the surface of thesintered bearing can be controlled easily.

Further, through the use of a mesh-belt type continuous furnace duringthe sintering step, productivity can be enhanced further and cost can bereduced further. From the viewpoint of a function of a sintered bearing,strength can be ensured sufficiently.

Specifically, the load on the mesh-belt type continuous furnace can bealleviated, and stable quality and manufacturing method can be realizedby setting the sintering temperature during the sintering step to from850 to 950° C., using a reducing atmosphere as the sintering atmosphere,and setting the sintering time to from 10 to 60 minutes.

Effects of the Invention

In the sintered bearing according to one embodiment of the presentinvention, corrosion resistance, mechanical characteristics such asstrength and abrasion resistance, oil film formation property, and oilholding property can be enhanced, and compactness and reduction in costcan be realized. Further, the manufacturing method for a sinteredbearing according to one embodiment of the present invention can realizea manufacturing method for an aluminum bronze-based sintered bearing,which has good productivity and low cost, and thus is suitable for massproduction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical sectional view of a sintered bearing according toeach of first to third embodiments of the present invention and asintered bearing based on a manufacturing method according to each offirst to third embodiments of the present invention.

FIG. 2 a is a schematic view of an enlarged metal structure in an Aportion of FIG. 1.

FIG. 2 b is a schematic view of an enlarged metal structure in a Bportion of FIG. 1.

FIG. 2 c is a schematic view of an enlarged metal structure in a Cportion of FIG. 1.

FIG. 3 is a diagram illustrating a manufacturing process for thesintered bearing.

FIG. 4 is a schematic view of a mixer for raw material powder.

FIG. 5 is a schematic view of a mesh-belt type continuous furnace.

FIG. 6 a is a view illustrating a sizing step.

FIG. 6 b is a view illustrating the sizing step.

FIG. 6 c is a view illustrating the sizing step.

FIG. 7 is a view illustrating a compressed state of a product during thesizing step.

FIG. 8 is a schematic view of an oil impregnation device.

FIG. 9 is a vertical sectional view of a fuel pump.

EMBODIMENTS OF THE INVENTION

Now, a sintered bearing according to a first embodiment of the presentinvention and a manufacturing method according to a first embodiment ofthe present invention are described with reference to the attacheddrawings. FIGS. 1 and 2 illustrate the sintered bearing according to thefirst embodiment, and FIGS. 3 to 8 illustrate the manufacturing methodaccording to the first embodiment.

As illustrated in FIG. 1, a sintered bearing 1 as the sintered bearingaccording to the first embodiment is formed in a cylindrical shapehaving a bearing surface 1 a on an inner circumference. When a shaft 2is inserted with respect to the inner circumference of the sinteredbearing 1, and the shaft 2 is rotated in this state, lubricating oilheld in an indefinite number of pores of the sintered bearing 1 seepsout from the bearing surface 1 a along with an increase in temperature.The lubricating oil that has seeped out forms an oil film between abearing gap between the outer circumferential surface of the shaft 2 andthe bearing surface 1 a, and the shaft 2 is supported by the bearing 1so as to be rotatable relatively.

The sintered bearing 1 according to the first embodiment is formed byfilling a form with raw material powder obtained by mixing variouspowders, compressing the raw material powder to form a green compact,and then sintering the green compact.

The raw material powder is mixed powder comprising copper powder, powderhaving a composition of copper, aluminum, and an aluminum-copper alloy,silicon powder, tin powder, phosphorus alloy powder, and graphite powderas main components. The detail of each powder is described below.

[Copper Powder]

As the copper powder, spherical or dendritic copper powder usedgenerally for a sintered bearing can be used, and for example, reducedpowder, electrolytic powder, water atomized powder, or the like is used.The grain size of the copper powder is set so that the copper powder ispowder having passed through a 100 mesh and a ratio of powder havingpassed through a 350 mesh is 40% or less.

[Powder Having Composition of Copper, Aluminum, and Aluminum-CopperAlloy]

The powder having a composition of copper, aluminum, and analuminum-copper alloy (hereinafter referred to as “aluminum-copper alloypowder) is obtained by heating mixed powder comprising 40 to 60% by massof aluminum alloy powder and the balance being copper powder in areducing or inert atmosphere, pulverizing the resultant, and adjustingthe grain size thereof. The preferred grain size of the aluminum-copperalloy powder is set so that the aluminum-copper alloy powder is powderhaving passed through a 80 mesh and a ratio of powder having passedthrough a 350 mesh is 60% or less. The problem of shortage of strengthof a green compact caused by a decrease in forming ability due to thehardness of powder is solved by using the aluminum-copper alloy powder,and in this case, there arises no problem of handling due to scatteringof aluminum simple substance particles having a small specific gravity.

It is preferred that the composition of the aluminum-copper alloy powderfall within a range of from 40 to 60% by mass of aluminum. When thepowder comprises aluminum in an amount of less than 40% by mass, thedensity of a green compact during pressure forming is decreased owing toa decrease in a copper powder mixing ratio, and the generation amount ofa copper silicon-based liquid phase under low-temperature sintering isreduced owing to the generation of an alloy phase having a high meltingpoint, with the result that the effect exhibited by adding a sinteringaccelerating element is decreased. On the other hand, when the powdercomprises aluminum in an amount of more than 60% by mass, unreactedaluminum particles are scattered owing to increases in aluminumparticles that remain unreacted with copper particles, and there arisesa problem of handling in terms of the scattering.

In the sintered bearing according to the first embodiment and themanufacturing method for the sintered bearing described later, copperpowder, aluminum-copper alloy powder, and phosphorus alloy powder,silicon powder, and tin powder described later are mixed at such a ratiothat the content of aluminum is from 3 to 10% by mass, the content ofsilicon is from 1 to 4% by mass, the content of tin is from 0.5 to 2% bymass, the content of phosphorus is from 0.05 to 0.5% by mass, and thebalance comprises copper as a main component, and graphite powder ismixed with a total of 100% by mass of the above-mentioned mixture sothat the blending amount of graphite is from 1 to 5% by mass to obtainraw material powder.

[Phosphorus Alloy Powder]

Phosphorus enhances wettability between a solid phase and a liquid phaseduring sintering and suppresses the generation of a nitride film thatshifts a temperature for generation of a liquid phase caused by theaddition of silicon powder to a low temperature side. It is preferredthat the blending amount of phosphorus be from 0.05 to 0.5% by mass.When the blending amount of phosphorus is less than 0.05% by mass, theaccelerating effect on the sintering between a solid phase and a liquidphase is unsatisfactory. On the other hand, when the blending amount ofphosphorus is more than 0.5% by mass, the sintering proceedsexcessively, and aluminum is segregated to increase a γ-phase, with theresult that a sintered compact becomes brittle.

[Silicon Powder]

Silicon is added as the sintering aid. Silicon generates a coppersilicon-based liquid phase with respect to a phase of inhibiting theprogress of sintering formed during the sintering step and acceleratesthe sintering. It is preferred that the blending amount of silicon befrom 1 to 4% by mass. When the blending amount of silicon is less than1% by mass, the amount of a liquid phase to be generated is small, andthe accelerating effect on liquid-phase sintering at low temperaturebecomes insufficient, with the result that a sintered compact that isdense and has appropriate hardness cannot be obtained. On the otherhand, when the blending amount of silicon is more than 4% by mass, thesintering proceeds excessively, and aluminum is segregated to increase aγ-phase, with the result that a sintered compact becomes brittle.

[Tin Powder]

Tin is added as the sintering aid. Tin has effects of compensating for adecrease in forming ability due to the addition of silicon powder anddecreasing a temperature for generation of a liquid phase generated bythe addition of silicon powder in the same way as in phosphorus. It ispreferred that the blending amount of tin be from 0.5 to 2% by mass.When the blending amount of tin be less than 0.5% by mass, the effect ofincreasing a green compact density by the addition of tin powder is notobtained. On the other hand, it is not preferred that the blendingamount of tin be more than 2% by mass, because a high concentration oftin is precipitated in a grain boundary, which degrades the quality ofouter appearance of a sintered compact.

[Graphite Powder]

Graphite is present mainly as free graphite in pores dispersed anddistributed in a substrate and contributes to the enhancement ofabrasion resistance by imparting excellent lubricity to a sinteredbearing. It is preferred that the blending amount of graphite be from 1to 5% by mass with respect to a total of 100% by mass of aluminum,silicon, tin, phosphorus, copper, and inevitable impurities. When theblending amount of graphite is less than 1% by mass, the effects ofenhancing lubricity and abrasion resistance by the addition of graphitecannot be obtained. On the other hand, it is not preferred that theblending amount of graphite be more than 5% by mass, because thestrength is decreased.

FIG. 2 are schematic views illustrating a metal structure in across-section of the sintered bearing according to the first embodiment.FIG. 2( a) is an enlarged view of an A portion of FIG. 1. Similarly,FIG. 2( b) is an enlarged view of a B portion of FIG. 1. FIG. 2( c) isan enlarged view of a C portion of FIG. 1. That is, FIG. 2( a)illustrates a metal structure of a surface layer portion of a bearingsurface on a radially inner side. FIG. 2( b) illustrates a metalstructure of an inside. FIG. 2( c) illustrates a metal structure of asurface layer portion of a radially outer surface. As illustrated inFIGS. 2( a), 2(b), and 2(c), an aluminum-copper alloy structure 3 is ahatched portion, and an aluminum oxide film 4 is present on a surfaceand around an inner pore. Therefore, the aluminum-copper alloy structureis excellent in corrosion resistance and abrasion resistance. Althoughnot shown, in a grain boundary portion of the aluminum-copper alloystructure 3, tin and phosphorus are present in large amounts and siliconis present in a scattered manner. Free graphite 5 is distributed in thepore, and hence the aluminum-copper alloy structure is excellent inlubricity and abrasion resistance.

As illustrated in FIG. 2( a), an open pore db1 formed on the bearingsurface on a radially inner side and an internal pore db2 in a surfacelayer of the bearing surface are formed. As illustrated in FIG. 2( b),pores di are formed in the bearing, and as illustrated in FIG. 2( c), anopen pore do1 formed on a radially outer surface and an internal poredo2 formed in a surface layer of the radially outer surface are formed.The open pore db1 formed on the bearing surface, the internal pore db2in the surface layer of the bearing surface, the pore di in the bearing,the open pore do1 formed on the radially outer surface, and the internalpore do2 formed in the surface layer of the radially outer surfacecommunicate with each other.

In the sintered bearing 1, both the radially outer surface 1 b of thebearing and the bearing surface 1 a on the radially inner side aresubjected to sizing processing after sintering in a manufacturing method(see FIG. 7) described later. In addition, an aluminum bronze-basedsintered bearing expands by sintering, and hence the radially outersurface 1 b of the bearing is sized in an amount larger than that of thebearing surface 1 a on the radially inner side. Therefore, the pores do(see FIG. 2( c)) in the surface layer portion on the radially outersurface 1 b side are more crushed than the pores db (see FIG. 2( a)) inthe surface layer portion on the bearing surface 1 a side. When the poredo in the surface layer portion on the radially outer surface 1 b side,the pore db in the surface layer portion on the bearing surface 1 aside, and the pore di (see FIG. 2( b)) in the bearing that is notcrushed are compared to each other in terms of size, a relationship ofdo<db<di is satisfied. By virtue of this relationship, corrosionresistance and oil film formation property can be enhanced on thebearing surface 1 a side. On the other hand, on the radially outersurface 1 b side and on the end surface 1 c side close to a closedstate, corrosion resistance and oil holding property can be enhanced.

The pores do, db, and di of the sintered bearing 1 are impregnated withlubricating oil. Thus, a satisfactory lubrication state can be obtainedfrom the beginning of an operation. As the lubricating oil, mineral oil,poly-α-olefin (PAO), an ester, liquid grease, or the like can be used.Note that, it is not necessarily required to impregnate those pores withthe lubricating oil in some use purposes of a bearing.

FIG. 1 illustrates a compressed layer in a surface layer of the sinteredbearing 1 with hatching. Hatching is provided only to an upper half in aradial direction of the bearing 1, and hatching is omitted in a lowerhalf thereof. The surface layer of the sintered bearing 1 has acompressed layer. A density ratio αo of a compressed layer Po in thesurface layer on the radially outer surface 1 b side and a density ratioαb of a compressed layer Pb in the surface layer on the bearing surface1 a side are both higher than a density ratio αi of the inside, and bothof the density ratios αo and αb are set in a range of 80%≦αo and αb≦95%.It is not preferred that the density ratios αo and αb be less than 80%,because the bearing strength becomes insufficient. On the other hand, itis not preferred that the density ratios αo and αb be more than 95%,because the oil content becomes insufficient. Herein, αo and αb arecollectively referred to as “α”.

In addition, when an average value of depths of the compressed layer Poin the surface layer on the radially outer surface 1 b side is definedas To, an average value of depths of the compressed layer Pb in thesurface layer on the bearing surface 1 a side is defined as Tb, andratios of To and Tb to the inner diameter dimension D1 of the bearingsurface are defined as To/D1 and Tb/D1, respectively, it is preferredthat relationships of 1/100≦To/D1 and Tb/D1≦ 1/15 be satisfied. Herein,the density ratio α is represented by the following expression:

α(%)=(ρ1/ρ0)×100

where ρ1 represents a density of a porous body, and ρ0 represents adensity in the case where it is assumed that the porous body has nomicropores.

It is not preferred that To/D1 and Tb/D1 be less than 1/100 becausepores are crushed insufficiently. On the other hand, it is not preferredthat To/D1 and Tb/D1 be more than 1/15 because pores are crushedexcessively. Note that, herein, To and Tb are collectively referred toas “T”.

Next, the manufacturing method for a sintered bearing according to thefirst embodiment is described. A sintered bearing is manufacturedthrough a raw material powder preparation step S1, a green compactforming step S2, a sintering step S3, a sizing step S4, and an oilimpregnation step S5 as illustrated in FIG. 3.

[Raw Material Powder Preparation Step S1]

In the raw material powder preparation step S1, raw material powder ofthe sintered bearing 1 is prepared and generated. The raw materialpowder was prepared and generated by adding 3% by mass of graphitepowder and 0.5% by mass of a lubricant such as zinc stearate or calciumstearate for enhancing forming ability to a total of 100% by mass of 81%by mass of copper powder, 12% by mass of 50% by mass aluminum-copperalloy powder, 3% by mass of silicon powder, 1% by mass of tin powder,and 3% by mass of 8% by mass phosphorus-copper alloy powder. By addingthe lubricant, a green compact described later can be released smoothly,and the deformation of the green compact due to the release can beprevented. Specifically, raw material powder M as described above issupplied to a can body 11 of a V-shaped mixer 10 illustrated in FIG. 4,and is uniformly mixed by rotating the can body 11.

[Green Compact Forming Step S2]

In the green compact forming step S2, the above-mentioned raw materialpowder is compressed to form a green compact 1′ (see FIG. 7) having ashape of the sintered bearing 1. The green compact 1′ is subjected tocompression forming so that a sintered compact 1″ to be formed byheating at a sintering temperature or more has a density ratio α of from70% to 80%. In FIG. 7, for convenience, the green compact and thesintered compact are illustrated with reference numerals 1′ and 1″,respectively.

Specifically, for example, a form having a cavity with a shape similarto that of a green compact is set on a CNC press machine using a servomotor as a drive source. The raw material powder filling the cavity iscompressed with a pressurization force of from 200 to 700 MPa to formthe green compact 1′. During forming of the green compact 1′, the formmay be heated to 70° C. or more.

In the manufacturing method for the sintered bearing 1 according to thefirst embodiment, the problem of shortage of strength of a green compactcaused by a decrease in forming ability due to fluidity is solved byusing aluminum-copper alloy powder as an aluminum source, and in thiscase, there arises no problem of handling due to scattering of aluminumsimple substance particles having a small specific gravity. Further, themanufacturing method has good production efficiency and hence issuitable for mass production.

[Sintering Step S3]

In the sintering step S3, the green compact 1′ is heated at a sinteringtemperature and the raw material powders adjacent to each other arebound by sintering to form the sintered compact 1″. The green compact 1′is supplied in a large amount to a mesh belt 16 of a mesh-belt typecontinuous furnace 15 illustrated in FIG. 5, and the green compact 1′ isheated at from 850 to 950° C. (for example, 900° C.) for 10 to 60minutes in a reducing atmosphere such as a mixed gas atmosphere ofnitrogen gas and hydrogen gas or a nitrogen gas atmosphere so as toprevent oxidation as much as possible, whereby the sintered compact 1″is formed. Thus, a load on the mesh-belt type continuous furnace isalleviated, and stable quality and manufacturing method can be realized.

The aluminum-copper alloy powder generates various liquid phases whenthe temperature is equal to or higher than 548° C. as the eutectictemperature. When the liquid phases are generated, the aluminum-copperalloy powder expands, and a sintering neck is formed owing to thegenerated liquid phases, which leads to densification, resulting inreduction in dimension. In the first embodiment, as a result of thesintering in the mesh-belt type continuous furnace 15, the surface ofthe sintered compact 1″ is oxidized, and the sintering thereof isinhibited. Therefore, the sintered compact 1″ is not densified, and thedimension thereof remains expanding. Note that, the inside of thesintered compact 1″ is sintered without being oxidized, and hence thestrength of the sintered compact 1″ can be ensured sufficiently. Owingto the use of the mesh-belt type continuous furnace 15, the sinteringtime from the supply of the green compact 1′ to the release thereof canbe shortened to mass-produce products, and cost can also be reduced.Further, from the viewpoint of the function of the sintered bearing, thestrength can be ensured sufficiently.

In the sintering step, the added phosphorus alloy powder, tin powder,silicon powder, and graphite powder exhibit the synergistic effectdescribed below. Thus, a sintered compact of good quality can be formed.First, phosphorus has effects of enhancing wettability between a solidphase and a liquid phase during sintering and shifting a temperature forgeneration of a liquid phase caused by the addition of silicon powder toa low temperature side, and hence a sintered compact of good quality canbe obtained. It is preferred that the blending amount of phosphorus befrom 0.05 to 0.5% by mass. When the blending amount of phosphorus isless than 0.05% by mass, the effect of accelerating the sinteringbetween a solid phase and a liquid phase is unsatisfactory. On the otherhand, when the blending amount of phosphorus is more than 0.5% by mass,a sintered compact to be obtained becomes brittle. Further, siliconserving as the sintering aid generates a copper silicon-based liquidphase with respect to a phase of inhibiting the progress of sinteringformed during the sintering step and accelerates the sintering. It ispreferred that the blending amount of silicon be from 1 to 4% by mass.When the blending amount of silicon is less than 1% by mass, the amountof a liquid phase to be generated is small and the effect ofaccelerating the sintering of a liquid phase at low temperature becomesinsufficient, with the result that a sintered compact that is dense andhas appropriate hardness cannot be obtained. On the other hand, when theblending amount of silicon is more than 4% by mass, a sintered compactto be obtained is hard and brittle.

In addition, tin serving as the sintering aid exhibits the effects ofcompensating for a decrease in forming ability due to the addition ofsilicon powder, and decreasing the temperature for generation of aliquid phase to be generated by the addition of silicon powder in thesame way as in phosphorus. It is preferred that the blending amount oftin be from 0.5 to 2% by mass. It is not preferred that the blendingamount of tin be less than 0.5% by mass, because the effect of raising agreen compact density by the addition of tin powder cannot be obtained.On the other hand, it is not preferred that the blending amount of tinbe more than 2% by mass, because a high concentration of tin isprecipitated in a grain boundary and inhibits the diffusion of aluminum.

The sintering aid is added to the raw material powder M as describedabove, and hence the sintered compact 1″ having a structure in which analuminum-copper alloy is sintered can be obtained, and strength andcorrosion resistance can be enhanced.

Further, graphite is present mainly as free graphite in pores dispersedand distributed in a substrate and contributes to the enhancement ofabrasion resistance by imparting excellent lubricity to a sinteredbearing. It is preferred that the blending amount of graphite be from 1to 5% by mass with respect to a total of 100% by mass of aluminum,silicon, tin, phosphorus, copper, and inevitable impurities. When theblending amount of graphite is less than 1% by mass, the effects ofenhancing lubricity and abrasion resistance by adding graphite powdercannot be obtained. On the other hand, it is not preferred that theblending amount of graphite be more than 5% by mass, because strength isdecreased.

[Sizing Step S4]

In the sizing step S4, the sintered compact 1″ that has expandedcompared to the green compact 1′ by sintering is subjected to dimensionshaping. FIG. 6 illustrate the details of the sizing step S4. A form forsizing processing includes a die 20, an upper punch 21, a lower punch22, and a core 23. As illustrated in FIG. 6( a), the sintered compact 1″is set on the lower punch 22 while the core 23 and the upper punch 21are retracted upward. As illustrated in FIG. 6( b), first, the core 23enters a radially inner portion of the sintered compact 1″. Then, asillustrated in FIG. 6( c), the sintered compact 1″ is pushed into thedie 20 by the upper punch 21. After that, the sintered compact 1″ iscompressed with the upper and lower punches 21, 22. As a result, thesurface of the sintered compact 1″ is subjected to dimension shaping.Pores in the surface layer of the expanding sintered compact 1″ arecrushed by sizing processing to cause a density difference between theinside of the product and the surface layer portion.

FIG. 7 illustrates a state in which the sintered compact 1″ iscompressed by sizing processing. The sintered compact 1″ before sizingprocessing is indicated by a two-dot chain line, and a product 1 aftersizing processing is indicated by a solid line. As indicated by thetwo-dot chain line, the sintered compact 1″ has expanded in a radialdirection and a width direction. Therefore, the radially outer surface 1b of the sintered compact 1″ is compressed more than the bearing surface1 a on the radially inner side. As a result, the pore do (see FIG. 2(c)) in the surface layer on the radially outer surface 1 b side iscrushed more than the pore db (see FIG. 2( a)) in the surface layer ofthe bearing surface 1 b on the radially inner side, and a relationshipof do<db<di is satisfied with respect to the pore di (see FIG. 2( b)) inthe bearing that is not crushed. By virtue of such relationship, in thebearing surface 1 a on the radially inner side, corrosion resistance andoil film formation property can be enhanced. On the other hand, in theradially outer surface 1 b and the end surface 1 c close to a closedstate, corrosion resistance and oil holding property can be enhanced.

The form used during the sizing step includes the die 20, a pair of thepunches 21, 22, and the core 23, and the radially inner side of thesintered compact 1″ is shaped by the core 23 by compressing the sinteredcompact 1″ from both sides in the axial direction and the radially outerside with the punches 21, 22 and the die 20. Thus, the sintered bearing1 can be subjected to dimension shaping and desired pores can be formedthrough effective use of the expansion of an aluminum bronze-basedsintered bearing by sintering.

Further, the size of each pore on the surface of the sintered compact 1″can be set by adjusting a dimension difference between the innerdiameter dimension of the die 20 and the outer diameter dimension of thesintered compact 1″ and a dimension difference between the outerdiameter dimension of the core 23 and the inner diameter dimension ofthe sintered compact 1″. Thus, the size of each pore on the surface ofthe sintered bearing 1 can be controlled easily.

[Oil Impregnation Step S5]

The oil impregnation step S5 is the step of impregnating the product 1(sintered bearing) with lubricating oil. FIG. 8 illustrates an oilimpregnation device. The product 1 is put in a tank 26 of the oilimpregnation device 25, and then lubricating oil 27 is poured into thetank 26. The inside of the tank 26 is reduced in pressure to impregnatethe pores do, db, and di (see FIG. 2) of the product 1 with thelubricating oil 27. Thus, a satisfactory lubrication state can beobtained from the beginning of an operation. As the lubricating oil,mineral oil, poly-α-olefin (PAO), an ester, liquid grease, or the likecan be used. Note that, it is appropriate that the pores do, db, and dibe impregnated with lubricating oil depending on the use purpose of abearing, and this impregnation is not necessarily required.

The sintered bearing 1 according to the first embodiment manufacturedthrough the above-mentioned steps has enhanced corrosion resistance,mechanical characteristics such as strength and abrasion resistance, oilfilm formation property, and oil holding property, and can also achievecompactness and reduction in cost.

Next, a sintered bearing according to a second embodiment of the presentinvention and a manufacturing method according to a second embodiment ofthe present invention are described. The second embodiment is differentfrom the first embodiment in that silicon and tin are used as thesintering aid in the first embodiment, whereas aluminum fluoride andcalcium fluoride are used as the sintering aid in the sintered bearingand manufacturing method in the second embodiment.

Raw material powder in the sintered bearing and manufacturing methodaccording to the second embodiment is mixed powder containing copperpowder, aluminum-copper alloy powder, phosphorus alloy powder, andgraphite powder used in the first embodiment, and a sintering aidcontaining aluminum fluoride and calcium fluoride as main components.Copper powder, aluminum-copper alloy powder, phosphorus alloy powder,and graphite powder are the same as those of the first embodiment, andhence repeated descriptions thereof are omitted.

In the second embodiment, copper powder, aluminum-copper alloy powder,and phosphorus alloy powder are mixed at such a ratio that the contentof aluminum is from 7 to 12% by mass, the content of phosphorus is from0.05 to 0.5% by mass, and the balance comprises copper as a maincomponent, and a total of 0.05 to 0.2% by mass of aluminum fluoride andcalcium fluoride serving as the sintering aid and 1 to 5% by mass ofgraphite are mixed with a total of 100% by mass of the above-mentionedmixture to obtain raw material powder.

[Aluminum Fluoride and Calcium Fluoride]

An aluminum oxide film to be generated on a surface ofaluminum-containing copper-based alloy powder during sintering inhibitsthe sintering remarkably. However, aluminum fluoride and calciumfluoride serving as the sintering aid evaporate gradually while beingmelted at a sintering temperature of from 850 to 900° C. of thealuminum-containing copper-based alloy powder and protect the surface ofthe aluminum-containing copper-based alloy powder to suppress thegeneration of aluminum oxide, thereby accelerating the sintering toincrease the diffusion of aluminum. Aluminum fluoride and calciumfluoride evaporate and volatilize during sintering, and hence hardlyremain in a finished product of the sintered bearing.

It is preferred that aluminum fluoride and calcium fluoride serving asthe sintering aid be added in an amount of from about 0.05 to 0.2% bymass in total with respect to a total of 100% by mass of raw materialpowder comprising aluminum, phosphorus, and the balance comprisingcopper as a main component, and inevitable impurities. When the additionamount of aluminum fluoride and calcium fluoride is less than 0.05% bymass, the effect as the sintering aid is insufficient, and a sinteredcompact that is dense and has appropriate hardness is not obtained. Onthe other hand, when the addition amount of aluminum fluoride andcalcium fluoride is more than 0.2% by mass, the effect as the sinteringaid is not enhanced even when they are added any more, and hence it ispreferred that the addition amount be limited to 0.2% by mass or lessfrom the viewpoint of cost.

The metal structure of a cross-section of the sintered bearing accordingto the second embodiment is the same as that of the first embodimentillustrated in the schematic view of FIG. 2. Therefore, only mainportions are described, and the repeated descriptions of the remainingportions are omitted. As illustrated in FIGS. 2( a), 2(b), and 2(c), inthe sintered bearing 1 according to the second embodiment, analuminum-copper alloy structure 3 is a hatched portion, and an aluminumoxide film 4 is present on a surface and around an internal pore.Therefore, the aluminum-copper alloy structure is excellent in corrosionresistance and abrasion resistance. Although not shown, in a grainboundary portion of the aluminum-copper alloy structure 3, phosphorus ispresent. Free graphite 5 is distributed in the pore, and hence thealuminum-copper alloy structure is excellent in lubricity and abrasionresistance.

In addition, in the sintered bearing 1 according to the secondembodiment as well, as illustrated in FIG. 7, both the radially outersurface 1 b of the bearing and the bearing surface 1 a on the radiallyinner side are subjected to sizing processing after sintering. Analuminum bronze-based sintered bearing expands by sintering, and hencethe radially outer surface 1 b of the bearing is sized in an amountlarger than that of the bearing surface 1 a on the radially inner side.Therefore, the pores do (see FIG. 2( c)) in the surface layer portion onthe radially outer surface 1 b side are more crushed than the pores db(see FIG. 2( a)) in the surface layer portion on the bearing surface 1 aside. When the pore do in the surface layer portion on the radiallyouter surface 1 b side, the pore db in the surface layer portion on thebearing surface 1 a side, and the pore di (see FIG. 2( b)) in thebearing which is not crushed are compared to each other in terms ofsize, a relationship of do<db<di is satisfied. By virtue of thisrelationship, corrosion resistance and oil film formation property canbe enhanced on the bearing surface 1 a side. On the other hand, on theradially outer surface 1 b side and on the end surface 1 c side close toa closed state, corrosion resistance and oil holding property can beenhanced. The pores do, db, and di of the sintered bearing 1 areimpregnated with lubricating oil. Thus, a satisfactory lubrication statecan be obtained from the beginning of an operation. As the lubricatingoil, mineral oil, poly-α-olefin (PAO), an ester, liquid grease, or thelike can be used. Note that, it is not necessarily required toimpregnate those pores with lubricant depending on the use purpose of abearing.

Further, the state of the compressed layer in the surface layer of thesintered bearing 1 according to the second embodiment is also the sameas that of the sintered bearing according to the first embodimentillustrated in FIG. 1. That is, as illustrated in FIG. 1, in thesintered bearing 1 according to the second embodiment, the surface layerof the sintered bearing 1 includes a hatched compressed layer. Accordingto the above-mentioned expression of the density ratio α, a densityratio αo of a compressed layer Po in the surface layer on the radiallyouter surface 1 b side and a density ratio αb of a compressed layer Pbin the surface layer on the bearing surface 1 a side are both higherthan a density ratio αi of the inside, and both of the density ratios αoand αb are set in a range of 80%≦αo and αb≦95%.

In addition, when an average value of depths of the compressed layer Poin the surface layer on the radially outer surface 1 b side is definedas To, an average value of depths of the compressed layer Pb in thesurface layer on the bearing surface 1 a side is defined as Tb, andratios of To and Tb to the inner diameter dimension D1 of the bearingsurface are defined as To/D1 and Tb/D1, respectively, To/D1 and Tb/D1are set to 1/100≦To/D1 and Tb/D1≦ 1/15.

The manufacturing method according to the second embodiment is also thesame as the manufacturing method for a sintered bearing according to thefirst embodiment illustrated in FIG. 3. Therefore, only differences ofspecific contents in the raw material powder preparation step S1 and thesintering step S3 are described.

[Raw Material Powder Preparation Step S1]

Raw material powder was prepared by adding a total of from 0.05 to 0.2%by mass of aluminum fluoride and calcium fluoride serving as thesintering aid, 1 to 5% by mass of graphite powder, and 0.5% by mass of alubricant such as zinc stearate or calcium stearate for enhancingforming ability to a total of 100% by mass of a remaining % by mass ofcopper powder, 14 to 20% by mass of 40 to 60% by mass aluminum-copperalloy powder, and 2 to 4% by mass of 8% by mass of phosphorus-copperalloy powder.

[Sintering Step S3]

In the sintering step, the added phosphorus alloy powder, aluminumfluoride, and calcium fluoride exhibit the effect described below. Thus,a sintered compact of good quality can be formed. First, phosphorus hasan effect of enhancing wettability between a solid phase and a liquidphase during sintering, and hence a sintered compact of good quality canbe obtained. It is preferred that the blending amount of phosphorus befrom 0.05 to 0.5% by mass. When the blending amount of phosphorus isless than 0.05% by mass, the effect of accelerating the sinteringbetween a solid phase and a liquid phase is unsatisfactory. On the otherhand, when the blending amount of phosphorus is more than 0.5% by mass,a sintered compact to be obtained becomes brittle. Further, aluminumfluoride and calcium fluoride serving as the sintering aid evaporategradually while being melted at a sintering temperature of from 850 to900° C. of the aluminum-containing copper-based alloy powder and protectthe surface of the aluminum-containing copper-based alloy powder tosuppress the generation of aluminum oxide, thereby enabling thesintering. Aluminum fluoride and calcium fluoride evaporate andvolatilize during sintering, and hence hardly remain in a finishedproduct of the sintered bearing. Note that, a green compact is sinteredin a case or the like because evaporation and volatilization occur. Itis preferred that aluminum fluoride and calcium fluoride serving as thesintering aid be added in an amount of from about 0.05 to 0.2% by massin total with respect to a total of 100% by mass of raw material powdercomprising aluminum, phosphorus, and the balance comprising copper as amain component, and inevitable impurities.

Also in the manufacturing method for a sintered bearing according to thesecond embodiment, in the same way as in the first embodiment, asillustrated in FIG. 6, the radially inner side of the sintered compact1″ is shaped by the core 23 by compressing the sintered compact 1″ fromboth sides in the axial direction and the radially outer side with thepunches 21, 22 and the die 20 during a sizing step. Thus, the sinteredbearing 1 can be subjected to dimension shaping and desired pores can beformed through effective use of the expansion of an aluminumbronze-based sintered bearing by sintering. Further, the size of eachpore on the surface of the sintered compact 1″ can be set by adjusting adimension difference between the inner diameter dimension of the die 20and the outer diameter dimension of the sintered compact 1″ and adimension difference between the outer diameter dimension of the core 23and the inner diameter dimension of the sintered compact 1″. Thus, thesize of each pore on the surface of the sintered bearing 1 can becontrolled easily.

Also in a sintered bearing based on the manufacturing method accordingto the second embodiment, corrosion resistance, mechanicalcharacteristics such as strength and abrasion resistance, oil filmformation property, and oil holding property can be enhanced, andcompactness and reduction in cost can be realized.

A sintered bearing according to a third embodiment of the presentinvention and a manufacturing method according to a third embodiment ofthe present invention are described. The sintered bearing according tothe third embodiment is specialized for a fuel pump of an automobileengine. The sintered bearing suppresses sulfidation corrosion andorganic acid corrosion caused by gasoline of inferior quality and isexcellent in performance such as initial conformability and durability.

FIG. 9 illustrates an example of a fuel pump for an automobile enginemounted with the sintered bearing according to the third embodimentincorporated therein. In a fuel pump 40, the sintered bearing 1 isprovided on the rotation side. Specifically, the fuel pump 40 includes acasing 41 having a liquid inlet 41 a and a liquid outlet 41 b, a fixedshaft 2 fixed to the casing 41 and protruding to an internal space ofthe casing 41, an impeller 42 provided rotatably with respect to thefixed shaft 2, a motor 43, a magnet 44 mounted on the impeller 42, and amagnet 45 mounted on a rotation shaft of the motor 43 and opposing in aradial direction to the magnet 44 on the impeller 42 side. The sinteredbearing 1 is fixed to an inner circumferential surface of the impeller42, and an inner circumferential surface (bearing surface 1 a, seeFIG. 1) of the sintered bearing 1 and an outer circumferential surfaceof the fixed shaft 2 are fitted to each other slidably in a rotationdirection. When the motor 43 is driven to rotate, the impeller 42rotates owing to attraction force between the magnet 45 on the motor 43side and the magnet 44 on the impeller 42 side. Thus, fuel having flowedinto the internal space of the casing 41 from the liquid inlet 41 a issent out from the liquid output 42 a.

In order to ensure performance such as initial conformability anddurability while suppressing sulfidation corrosion and organic acidcorrosion caused by gasoline of inferior quality in an environmentconstantly held in contact with the gasoline 34 as described above,various studies and test evaluation were conducted, and the thirdembodiment was achieved based on the following findings.

(1) In a relationship between the blending amount of aluminum andcorrosion resistance, when the amount of aluminum increases, thediffusion of aluminum into copper increases, and thus the effect ofcorrosion resistance is large.

(2) In a relationship between the sintering temperature and thecorrosion resistance, when the sintering temperature is increased, thediffusion of aluminum increases, and thus the effect of corrosionresistance is large.

(3) In a relationship between the density of a sintered bearing and thecorrosion resistance, when the density increases, the effect ofcorrosion resistance is slightly enhanced.

(4) The additives (phosphorus, zinc, and silicon) can accelerate thediffusion of aluminum during the sintering step to reduce the amount ofaluminum, thereby reducing the precipitation of a γ-phase of an aluminumstructure that degrades corrosion resistance and initial conformability.

In order to suppress sulfidation corrosion and organic acid corrosionand enhance performance such as initial conformability and durability ina sintered bearing for a fuel pump, in the sintered bearing andmanufacturing method according to the third embodiment, copper powder,aluminum-copper alloy powder, phosphorus alloy powder, silicon powder,and zinc alloy powder are mixed at such a ratio that the content ofaluminum is from 8 to 9% by mass, the content of phosphorus is from 0.05to 0.5% by mass, the content of silicon is from 0.5 to 3% by mass, thecontent of zinc is from 2 to 4% by mass, and the balance comprisescopper as a main component, and a total of 0.05 to 0.2% by mass ofaluminum fluoride and calcium fluoride, and 1 to 5% by mass of graphiteare mixed with a total of 100% by mass of the above-mentioned mixture toobtain raw material powder. Regarding the raw material powder, althoughsome parts are similar to those of the first and second embodiments, thedetails of each powder are described below.

[Copper Powder]

As the copper powder, there are given atomized powder, electrolyticpowder, and pulverized powder. However, in order to sufficiently diffusealuminum into copper, dendritic electrolytic powder is effective and isexcellent in forming ability, sintering property, and slidingcharacteristics. Therefore, in the third embodiment, electrolytic powderwas used as copper powder. The grain size thereof is set so that theelectrolytic powder is powder having passed through a 100 mesh and aratio of powder having passed through a 350 mesh is 40% or less.

[Aluminum-Copper Alloy Powder]

50% by mass aluminum-copper alloy powder was pulverized to adjust agrain size thereof. The preferred grain size of the aluminum-copperalloy powder is set so that the aluminum-copper alloy powder is powderhaving passed through 145 mesh and a ratio of powder having passedthrough a 350 mesh is 60% or more. The use of the aluminum-copper alloypowder extracts the effect of additives such as graphite, phosphorus,and zinc, and enables a sintered bearing material to be excellent in,for example, corrosion resistance, strength, and slidingcharacteristics. Further, the aluminum-copper alloy powder is alloypowder, and hence there is no problem of handing due to scattering ofaluminum simple substance powder having a small specific gravity.

In the aluminum structure, an α-phase is most excellent in corrosionresistance with respect to sulfidation corrosion and organic acidcorrosion, and initial conformability. When 50% by mass aluminum-copperalloy powder is used, even when graphite is added, the strength isobtained, and a sintered bearing can be manufactured. When the structureis a γ-phase, although excellent abrasion resistance is obtained,corrosion resistance and initial conformability are degraded.

In addition, in order to accelerate the sintering of copper andaluminum, phosphorus alloy powder, zinc alloy powder, and a fluoride(aluminum fluoride, calcium fluoride) are added. Thus, the diffusion ofaluminum with respect to copper is enhanced during liquid-phasesintering and solid-phase sintering. When the diffusion of aluminum isaccelerated, corrosion resistance is enhanced.

[Phosphorus Alloy Powder]

AS the phosphorus alloy powder, in the same way as in the first andsecond embodiments, 8% by mass phosphorus-copper alloy powder was used.Phosphorus has effects of enhancing wettability between a solid phaseand a liquid phase during sintering and shifting a temperature forgeneration of a liquid phase caused by the addition of silicon powder toa low temperature side. It is preferred that the blending amount ofphosphorus be from 0.05 to 0.5% by mass. When the blending amount ofphosphorus is less than 0.05% by mass, the effect of accelerating thesintering between a solid phase and a liquid phase is unsatisfactory. Onthe other hand, when the blending amount of phosphorus is more than 0.5%by mass, the sintering proceeds excessively, and aluminum is segregatedto increase γ-phase precipitation, with the result that a sinteredcompact becomes brittle.

[Zinc Alloy Powder]

As the zinc alloy powder, zinc-copper alloy powder was used. Zinc has alow melting point, accelerates the sintering of copper and aluminum, andalso accelerates the diffusion of aluminum. Further, zinc is excellentin corrosion resistance.

It is preferred that the blending amount of zinc be from 1% by mass to5% by mass. When the blending amount of zinc is less than 1% by mass,the effects of accelerating the sintering of copper and aluminum andaccelerating the diffusion of aluminum are not obtained. On the otherhand, when the blending amount of zinc is more than 5% by mass, zincevaporates to contaminate a sintering furnace during sintering, andsintering proceeds excessively, with the result that aluminum issegregated and the diffusion of aluminum is inhibited.

[Silicon Powder]

Silicon is added as the sintering aid. Silicon generates a coppersilicon-based liquid phase with respect to a phase of inhibiting theprogress of sintering formed during the sintering step and acceleratesthe sintering. Silicon can accelerate the diffusion of aluminum duringthe sintering step to reduce the amount of aluminum, thereby reducing aγ-phase. It is preferred that the blending amount of silicon be from 0.5to 3% by mass. When the blending amount of silicon is less than 0.5% bymass, the effect of accelerating the diffusion of aluminum during thesintering step is insufficient, and along with this, the effect ofreducing a γ-phase becomes insufficient. On the other hand, when theblending amount of silicon is more than 3% by mass, silicon reactsduring an increase in temperature of sintering to oxidize aluminum inblack, which causes inconvenience of discoloration.

[Graphite Powder]

Graphite is present mainly as free graphite in pores dispersed anddistributed in a substrate and contributes to the enhancement ofabrasion resistance by imparting excellent lubricity to a sinteredbearing. It is preferred that the blending amount of graphite be from 1to 5% by mass with respect to a total of 100% by mass of aluminum,silicon, tin, phosphorus, copper, and inevitable impurities. When theblending amount of graphite is less than 1% by mass, the effects ofenhancing lubricity and abrasion resistance by the addition of graphitecannot be obtained. On the other hand, it is not preferred that theblending amount of graphite be more than 5% by mass, because thestrength is decreased.

[Aluminum Fluoride and Calcium Fluoride]

Aluminum fluoride and calcium fluoride are the same as those of thesecond embodiment, and hence the repeated descriptions thereof areomitted.

The metal structure of a cross-section of the sintered bearing accordingto the third embodiment is also the same as those of the first andsecond embodiments illustrated in the schematic view of FIG. 2.Therefore, only main portions are described, and the repeateddescription of the remaining portions is omitted. In the sinteredbearing 1 according to the third embodiment, as illustrated in FIGS. 2(a), 2(b), and 2(c), an aluminum-copper alloy structure 3 is a hatchedportion, and an aluminum oxide film 4 is present on a surface and aroundan inner pore. Therefore, the aluminum-copper alloy structure isexcellent in corrosion resistance and abrasion resistance. Although notshown, in a grain boundary portion of the aluminum-copper alloystructure 3, phosphorus is present. Free graphite 5 is distributed inthe pore, and hence the aluminum-copper alloy structure is excellent inlubricity and abrasion resistance.

Further, in the sintered bearing 1 according to the third embodiment aswell, as illustrated in FIG. 7, both the radially outer surface 1 b ofthe bearing and the bearing surface 1 a on the radially inner side aresubjected to sizing processing after sintering. An aluminum copper-basedsintered bearing expands by sintering, and hence the radially outersurface 1 b of the bearing is sized in an amount larger than that of thebearing surface 1 a on the radially inner side. Therefore, the pores do(see FIG. 2( c)) in the surface layer portion on the radially outersurface 1 b side are more crushed than the pores db (see FIG. 2( a)) inthe surface layer portion on the bearing surface 1 a side. When the poredo in the surface layer portion on the radially outer surface 1 b side,the pore db in the surface layer portion on the bearing surface 1 aside, and the pore di (see FIG. 2( b)) in the bearing that is notcrushed are compared to each other in terms of size, a relationship ofdo<db<di is satisfied. By virtue of this relationship, corrosionresistance and oil film formation property can be enhanced on thebearing surface 1 a side. On the other hand, on the radially outersurface 1 b side and on the end surface 1 c side close to a closedstate, corrosion resistance and oil holding property can be enhanced.The pores do, db, and di of the sintered bearing 1 are impregnated withlubricating oil. Thus, a satisfactory lubrication state can be obtainedfrom the beginning of an operation. As the lubricating oil, mineral oil,poly-α-olefin (PAO), an ester, liquid grease, or the like can be used.Note that, it is not necessarily required to impregnate those pores withthe lubricating oil depending on the use purpose of a bearing.

Further, the state of the compressed layer of the surface layer of thesintered bearing 1 according to the third embodiment is also the same asthat of the sintered bearing according to each of the first and secondembodiments illustrated in FIG. 1. That is, as illustrated in FIG. 1, inthe sintered bearing 1 according to the third embodiment as well, thesurface layer of the sintered bearing 1 includes a hatched compressedlayer. According to the above-mentioned expression of the density ratioα, a density ratio αo of a compressed layer Po in the surface layer onthe radially outer surface 1 b side and a density ratio αb of acompressed layer Pb in the surface layer on the bearing surface 1 a sideare both higher than a density ratio αi of the inside, and both of thedensity ratios αo and αb are set in a range of 80%≦αo and αb≦95%.

In addition, when an average value of depths of the compressed layer Poin the surface layer on the radially outer surface 1 b side is definedas To, an average value of depths of the compressed layer Pb in thesurface layer on the bearing surface 1 a side is defined as Tb, andratios of To and Tb to the inner diameter dimension D1 of the bearingsurface are defined as To/D1 and Tb/D1, respectively, To/D1 and Tb/D1are set to 1/100<To/D1 and Tb/D1< 1/15.

The manufacturing method according to the third embodiment is also thesame as the manufacturing method for a sintered bearing according toeach of the first and second embodiments illustrated in FIG. 3.Therefore, only differences of specific contents in the raw materialpowder preparation step S1, the sintering step S3, and the sizing stepS4 are described.

[Raw Material Powder Preparation Step S1]

Raw material powder was prepared by adding a total of 0.05 to 0.2% bymass of aluminum fluoride and calcium fluoride serving as the sinteringaid, 1 to 5% by mass of graphite powder, and 0.5% by mass of a lubricantsuch as zinc stearate or calcium stearate for enhancing forming abilityto a total of 100% by mass of a remaining % by mass of electrolyticcopper powder, 14 to 20% by mass of 40 to 60% by mass aluminum-copperalloy powder, 2 to 4% by mass of 8% by mass of phosphorus-copper alloypowder, 1 to 3% by mass of silicon powder, and 6 to 8% by mass of 20 to40% by mass zinc-copper alloy powder.

[Sintering Step S3]

What is important in the sintering step is to enhance corrosionresistance by sufficiently diffusing aluminum into copper, and toenhance corrosion resistance and bearing performance (initialconformability) by setting an aluminum structure to an α-phase. When thealuminum structure is a γ-phase, although abrasion resistance isexcellent owing to the enlarged hardness, corrosion resistance isdegraded. Therefore, it was found that it was necessary to reduce theamount of aluminum so as to minimize the precipitation of a γ-phase.

As the sintering condition satisfying the foregoing, it is preferredthat the sintering temperature be from 900 to 950° C. Further, in asintered bearing for a fuel pump, it is preferred that the sinteringtemperature be from 900 to 920° C. (for example, 920° C.). Further, asatmospheric gas, hydrogen gas, nitrogen gas, or mixed gas hereof isused. As the sintering time is longer, corrosion resistance becomes moresatisfactory, and in a sintered bearing for a fuel pump, the sinteringtime is preferably 20 to 60 minutes (for example, 30 minutes).

[Sizing Step S4]

Also in the manufacturing method for a sintered bearing according to thethird embodiment, in the same way as in the first and secondembodiments, as illustrated in FIG. 6, the radially inner side of thesintered compact 1″ is shaped by the core 23 by compressing the sinteredcompact 1″ from both sides in the axial direction and the radially outerside with the punches 21, 22 and the die 20 during a sizing step. Thus,the sintered bearing 1 can be subjected to dimension shaping and desiredpores can be formed through effective use of the expansion of analuminum bronze-based sintered bearing by sintering. Further, the sizeof each pore on the surface of the sintered compact 1″ can be set byadjusting a dimension difference between the inner diameter dimension ofthe die 20 and the outer diameter dimension of the sintered compact 1″and a dimension difference between the outer diameter dimension of thecore 23 and the inner diameter dimension of the sintered compact 1″.Thus, the size of each pore on the surface of the sintered bearing 1 canbe controlled easily. Further, although not shown, each pore of thebearing surface 1 a can be reduced in size by subjecting the bearingsurface 1 a (see FIG. 7) to rotation sizing.

The sintered bearing based on the manufacturing method according to thethird embodiment has enhanced corrosion resistance, mechanicalcharacteristics such as strength and abrasion resistance, oil filmformation property, and oil holding property, and can also achievecompactness and reduction in cost. In particular, in a sintered bearingfor a fuel pump, sulfidation corrosion and organic acid corrosion causedby gasoline of inferior quality are suppressed, and excellentperformance such as initial conformability and durability are realized.

As the purpose of the sintered bearings according to the above-mentionedembodiments, a fuel pump has been illustrated. However, the purpose isnot limited thereto. For example, the present invention can beappropriately applied to bearings required to have corrosion resistance,such as an exhaust gas recirculation device (EGR) and a fishing reel.

In the above-mentioned description of each embodiment, the case wherethe present invention is applied to a perfect circular bearing havingthe bearing surface 1 a in a perfect circle shape has been illustrated.However, the present invention can be similarly applied to a fluiddynamic bearing in which a dynamic pressure generation portion such as aherring-bone groove or a spiral groove is provided on the bearingsurface 1 a and an outer circumferential surface of the shaft 2, as wellas the perfect circular bearing.

The present invention is not limited to the above-mentioned embodiments.As a matter of course, various modifications can be made thereto withoutdeparting from the gist of the present invention. The scope of thepresent invention is defined in claims, and encompasses equivalentsdescribed in claims and all changes within the scope of claims.

DESCRIPTION OF REFERENCE SIGNS

-   -   1 sintered bearing    -   1′ green compact    -   1″ sintered compact    -   1 a bearing surface    -   1 b radially outer surface    -   1 c end surface    -   2 shaft    -   3 aluminum-copper alloy structure    -   4 aluminum oxide film    -   5 free graphite    -   15 mesh-belt type continuous furnace    -   20 die    -   21 upper punch    -   22 lower punch    -   23 core    -   40 fuel pump    -   D1 inner diameter dimension of bearing surface    -   db pore    -   di pore    -   do pore    -   Ti compressed layer    -   To compressed layer

1. A sintered bearing, comprising 3 to 12% by mass of aluminum, 0.05 to0.5% by mass of phosphorus, and the balance comprising copper as a maincomponent, and inevitable impurities, the sintered bearing having astructure in which an aluminum-copper alloy is sintered with a sinteringaid added to raw material powder, a pore in a surface layer portion ofthe sintered bearing being formed smaller than an internal pore.
 2. Thesintered bearing according to claim 1, wherein the sintered bearing hasadded thereto, as the sintering aid, 1 to 4% by mass of silicon and 0.5to 2% by mass of tin with respect to the raw material powder.
 3. Thesintered bearing according to claim 1, wherein the sintered bearing hasadded thereto, as the sintering aid, a total of 0.05 to 0.2% by mass ofaluminum fluoride and calcium fluoride with respect to a total of 100%by mass of the raw material powder comprising aluminum, phosphorus, andthe balance comprising copper as a main component, and the inevitableimpurities.
 4. The sintered bearing according to claim 3, wherein thesintered bearing has added thereto 2 to 4% by mass of zinc.
 5. Thesintered bearing according to claim 3, wherein the sintered bearing hasadded thereto 0.5 to 3% by mass of silicon.
 6. The sintered bearingaccording to claim 1, wherein the copper in the raw material powdercomprises electrolytic copper powder as a main component.
 7. Thesintered bearing according to claim 1, wherein the sintered bearing hasadded thereto 1 to 5% by mass of graphite with respect to a total of100% by mass of the raw material powder and the inevitable impurities.8. The sintered bearing according to claim 1, wherein: the sinteredbearing has a compressed layer in a surface layer; a density ratio (α1)of the compressed layer is higher than a density ratio (α2) of aninside; the density ratio (α1) is 80%≦α1≦95; and a ratio (T/D1) of anaverage value (T) of depths of the compressed layer and an innerdiameter dimension (D1) of a bearing surface is 1/100≦T/D1≦ 1/15.
 9. Thesintered bearing according to claim 1, wherein, in an outer surface ofthe sintered bearing, a pore on a bearing surface is formed larger thana pore on the remaining outer surface.
 10. The sintered bearingaccording to claim 1, wherein the sintered bearing comprises anoil-impregnated bearing.
 11. The sintered bearing according to claim 1,wherein the sintered bearing is used for a fuel pump and comprisesaluminum in an amount of from 8 to 9% by mass.
 12. A manufacturingmethod for a sintered bearing comprising 3 to 12% by mass of aluminum,0.05 to 0.5% by mass of phosphorus, and the balance comprising copper asa main component, and inevitable impurities, the manufacturing methodcomprising at least: a green compact forming step of forming a greencompact in which a sintering aid is added to raw material powder; asintering step of obtaining, from the green compact, a sintered compacthaving a structure in which an aluminum-copper alloy is sintered; and asizing step of subjecting the sintered compact to dimension shaping. 13.The manufacturing method for a sintered bearing according to claim 12,wherein the sintered bearing has added thereto, as the sintering aid, atotal of 0.05 to 0.2% by mass of aluminum fluoride and calcium fluoridewith respect to a total of 100% by mass of the raw material powdercomprising aluminum, phosphorus, and the balance comprising copper as amain component, and the inevitable impurities.
 14. The manufacturingmethod for a sintered bearing according to claim 12, wherein the copperin the raw material powder comprises electrolytic copper powder as amain component.
 15. The manufacturing method for a sintered bearingaccording to claim 12, wherein the sintered bearing has added thereto 1to 5% by mass of graphite with respect to a total of 100% by mass of theraw material powder and the inevitable impurities.
 16. The manufacturingmethod for a sintered bearing according to claim 12, wherein: a form inthe sizing step comprises a die, a pair of punches, and a core; and thesizing step comprises compressing the sintered compact from both sidesin an axial direction and a radially outer side with the punches and thedie to shape a radially inner side of the sintered compact with thecore.
 17. The manufacturing method for a sintered bearing according toclaim 12, wherein the sizing step comprises setting a size of a pore ona surface of the sintered compact by adjusting a dimension differencebetween an inner diameter dimension of the die and an outer diameterdimension of the sintered compact and a dimension difference between anouter diameter dimension of the core and an inner diameter dimension ofthe sintered compact.
 18. The manufacturing method for a sinteredbearing according to claim 12, wherein the sintering step comprisesusing a mesh-belt type continuous furnace.
 19. The manufacturing methodfor a sintered bearing according to claim 12, wherein a sinteringtemperature of the sintering step is from 850 to 950° C.
 20. Themanufacturing method for a sintered bearing according to claim 12,wherein: a sintering atmosphere of the sintering step comprises areducing atmosphere; and a sintering time of the sintering step is from10 to 60 minutes.